Pastels have been my weapon of choice for weathering and for a long time I used nothing but. I’ve never applied pastels directly to a model by rubbing the stick or block onto it, I’ve always used a specific selection of brushes.
Here’s some examples of pastels at work on a model:

Direct Link

Direct Link
Sorry for the poor photography, I ain’t my Dad!!! Now there was a photographer…
Anyhoo, this Hurri was painted overall black and any tonal variations you see (which are, unfortunately, mostly washed out in these shots) were achieved using pastels.

Direct Link
This thing was built over ten years ago before I had learned the ‘subtle’ art of weathering! [:p] So it’s a bit heavy handed in many ways, especially the paint chipping and scuffs and so forth. But it’s an excellent example of the use of pastels…
I had painted this thing in a scheme which was supposed to be BraunViolet on the upper surfaces. At the time, the ‘new’ formula of Micro Flat had been released and I decided to give it a try, finding out the hard way that even applied in light coats it had adverse effects on a model. In this case, the entire thing turned a color which was akin to a washed out Panzer Gray!! Needless to say I was pi**ed and in the mood to firebomb Microscale Industries. In the end it was probably my relative inexperience at the time which caused this, but…
Since all the color was gone from my 262 I decided to experiment a bit, so both the tonal variations and all the colors you see on this model were achieved using pastels. Anyhoo, it’s since become a testbed and I’ll use it for trying out various things now and again…
I mentioned a specific selection of brushes; I use only what are known as ‘Cat’s Tongue’ brushes. These are fairly wide brushes which have a thin cross-section and which are rounded at the top, ie; they look something like a cat’s tongue. These brushes are a good compromise between firmness and flexibility of the bristles and are quite diverse.
As to technique, I wrote an article years ago explaining the techniques which I had developed and if I can dig it up I may post it on the forum somewhere. This bit about rubbing the chalk into the finish with any sort of abrasive paper, especially the finer grits, isn’t as horrifying as it sounds; it makes sense, actually. Although I have never tried it myself…
In the end, though, the best advice anyone can give is to experiment and practice with a number of techniques and see what you like best…
Fade to Black…